Maxwell's Thermodynamic Surface

ideal gas

U=0.75N53h22S-53NkBV23mπ

van der Waals

U=0.75N53h2-53+2S3NkBmπ-Nb+V23-1.0N2aV

toy model

U=0.375N53h2-53+2S3NkBmπ-Nb+V23-0.5N2aV+250.00.00181880889615721100000.0V+170-S+1